Proper regulation of MHC class II molecules is crucial to the initiation and maintenance of healthy immune responses. Defining the molecular mechanisms governing their regulation is important for understanding immune responses to infection, autoimmunity, tumorigenesis, and transplantation. We demonstrated that CREB, a factor positively associated with genes that are activated in response to cAMP, was involved in regulating MHC class H genes. This finding presented a paradox, as increases in cAMP levels are known to down modulate MHC class II gene expression. Phosphorylation of CREB in response to increases in cAMP levels allows CREB to interact with CBP, a transcriptional coactivator that has the ability to acetylate histones. CREB also interacts directly with CHTA, an MHC class H specific coactivator that can bind CBP on its own. The relationship between CREB, CBP, and CIITA is not known. Aims 1 and 2 will attempt to elucidate this relationship and explain the paradox. The potential for histone acetylation at MHC class H genes prompted us to investigate whether changes in chromatin structure of the HLA-DRA gene occur during distinct modes of regulation. The observed changes couple CHTA to chromatin acetylation and indicate an additional level of control that has been previously unexplored. Aim 3 focuses on characterizing changes in nucleosome acetylation at the DRA gene and will attempt to link these changes to the properties of the recruited factors. To explore the role of chromatin acetylation further, a search of the completed MHC sequences was conducted for MHC class H regulatory regions termed X-Y boxes. A plethora of X-Y sequences were found that were not associated with genes. What are the roles of these "orphan" sequences? If these sequences function like the classical X-Y box elements, then factor assembly and subsequent nucleosome acetylation may take place across the class II region, providing a global "opening" of the class II region during MHC class H gene expression. Aim 4 will focus on examining global changes in chromatin structure within the class H region of the MHC and the potential role that these orphan X-Y box elements have on the regulation of non-MHC class II genes. Results from this study should begin to integrate a model of MHC class H expression with specific factors and the global regulation of the MHC.